SOME POINTS IN THE ANATOMY OF POLYOH.ETA. 275 



It would be very startling, if not even absurd, to maintain 

 tbat such neural canals as are seen in Sabella, and in Nerine 

 are colossal nerve-fibres, and that their contents form a nervous 

 medulla which commences as a process from a ganglion cell. I 

 have entirely failed to trace any connection between these 

 canals and any ganglion cells. At the same time it is difficult 

 to refuse to admit that the neural canals of the Sedentaria are 

 completely homologous with those of the Errantia. 



In my opinion, in both cases they are supporting structures 

 which serve to prevent the nerve-cords being bent at a sharp 

 angle, causing them always to remain in curves, and so to 

 escape injury during the wriggling and burrowing of the worm. 

 It is noticeable as a support of this view that the canals always 

 reach their greatest development in worms which are extremely 

 long in proportion to their thickness. Sabella and Nerine are 

 both extremely long, as compared, for instance, to Ophelia or 

 Cirratulus. Another fact, which seems to have some signifi- 

 cance, is that where the neural canals show their maximum 

 development the nerve-cord is not separated from the epi- 

 dermis, and is therefore more exposed to the danger of being 

 injured than where they have reached a more internal position. 

 The origin of the vertebrate notochord from the hypoblast 

 seems so well established that a comparison of it with the 

 neural canals of the Chaetopoda will scarcely be regarded 

 seriously by morphologists. At the same time, seeing that the 

 notochord at a later stage is separated from the iutestine by 

 the aorta, it is very difficult to understand how the former 

 structure could have been derived phylogenetically from the 

 intestine. The neural canals are remarkably constant through- 

 out the Chaetopoda, those in the Polychseta being obviouslv 

 homologous with the three giant-fibres in the earthworm and 

 other Oligochseta. They have a position in relation to the 

 nerve-cords and ventral blood-vessel which is similar to that of 

 the notochord in relation to the neurochord and aorta. Their 

 origin in the embryo has not so far as I know been investi- 

 gated, so that it is doubtful whether they are intercellular or 

 intracellular in origin. I hope to devote further attention to 



